mesa/util: add allow_glsl_relaxed_es driconfig override

mesa/util: add allow_glsl_relaxed_es driconfig override

This relaxes a number of ES shader restrictions allowing shaders
to follow more desktop GLSL like rules.

This initial implementation relaxes the following:

 - allows linking ES shaders with desktop shaders
 - allows mismatching precision qualifiers
 - always enables standard derivative builtins

These relaxations allow Google Earth VR shaders to compile.

Reviewed-by: Dave Airlie <airlied@redhat.com>